1. Field of the Invention
The present invention relates to a side-face illuminating optical fiber utilizing an optical fiber formed of a core and a cladding, and in particular, to a side-face illuminating optical fiber which can be used for solar light transmission, illumination for automobile components, illumination at places equipped with a water supply at washstands, kitchens and bathrooms, illumination at handrails, corridors, stairs and the like in a senior-friendly house, and a light guide using illumination cutting off infrared-rays or ultraviolet-rays at art galleries and museums.
2. Description of the Related Art
A glass fiber has a core and a cladding that covers the core. Conventionally, as a material for a core of an optical fiber, there have been used a glass such as quartz glass or multi-component glass, an acryl or styrol-based plastics such as methylmethacrylate and a transparent liquid such as tetrachloroethylene. For the cladding, there have been used soda lime glasses and boro-silicate glasses with a refractive index lower than that of the core, and fluoro resins such as tetra-fluoro ethylene/polyfluoro vinylidene copolymer.
Some parties have attempted to utilize an optical fiber for illumination or the like by illuminating with light from its side face. Such an optical fiber is hereinafter referred to as side-face illuminating optical fiber. Light transmitted through the core is allowed to be scattered through the interface between the core and the cladding and to illuminate with the scattered light from the side face of the optical fiber.
However, the optical fiber has a limitation in the quantity of light to be condensed, and therefore, the quantity of light of the illumination from the side face is insufficient, as will be explain below. Therefore, the side-face illuminating optical fiber is still far from becoming part of the mainstream of illumination.
In the optical fiber, the refractive index of the cladding is lower than that of the core. The acceptance angle of the optical fiber and the total reflection angle at the interface between the core and the cladding vary with difference in refractive index between the core and the cladding. In general, the relative index difference expressed by the following equation is used to represent the difference in refractive index between the core and the cladding. EQU Relative index difference=(n.sub.1 -n.sub.2)/n.sub.1,
where n.sub.1 represents the refractive index of the core and n.sub.2 represents that of the cladding.
The numerical aperture and the acceptance angle .theta. (see FIG. 1) of the optical fiber are expressed by the following equation. EQU Numerical aperture=n.multidot.sin .theta.=(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2,
where n represents the refractive index at the outside of the optical fiber and n of air is normally 1.0.
As is observed in these equations, the acceptance angle .theta. of the optical fiber increases with increase in the difference in refractive index between the core and the cladding. That is, the greater the relative index difference is, the greater the angle is. In order to condense and transmit a larger quantity of light, it is required to increase the relative index difference or to the increase the acceptance angle .theta. of the optical fiber. This can be achieved by increasing the refractive index of the core and by reducing that of the cladding.
In the optical fiber, pure quartz glass, which has a small optical loss, excellent heat resistance and chemical resistance, is often used as the core. However, the quartz glass has a low refractive index of 1.46, it is a problem to select a cladding having a lower refractive index. Therefore, when glass is used for the cladding, a component such as B.sub.2 O.sub.3, fluorine or the like is added to reduce refractive index in order to reduce a refractive index from that of pure quartz glass. In order to increase the refractive index while maintaining low optical loss, a dopant to increase refractive index may be added quartz glass. As such a dopant, there can be enumerated TiO.sub.2, Ta.sub.2 O.sub.5, SnO.sub.2, Nb.sub.2 O.sub.5, ZrO.sub.2, Yb.sub.2 O.sub.3, La.sub.2 O.sub.3 and Al.sub.2 O.sub.3. In this case, pure quartz glass or doped quartz glass having a lower refractive index can be used as the cladding. When using a plastics material as the cladding, there is employed silicon resin such as polysiloxane or silicone rubber, fluorine containing resin such as fluorinated ethylene propylene or polyvinylidene fluoride, but these materials have a low refractive index of about 1.29 to 1.33.
As stated above, the acceptance angle .theta. of the optical fiber varies with the relative index difference between the core and the cladding. For example, with regard to the light guide, if flint F2 glass (having refractive index of 1.62) is used for the core and soda-lime glass (having refractive index of 1.52) is used for the cladding, then the numerical aperture becomes 0.56 and the acceptance angle .theta. becomes 34.degree.. With regard to the plastics optical fiber, if methacrylic resin (having refractive index of 1.49) is used for the core and fluororesin (having refractive index of 1.39) is used for the cladding, then the numerical aperture becomes 0.54 and the acceptance angle .theta. becomes 32.degree.. As described above, in the case where an optical fiber is manufactured with the use of the conventional core and cladding, the acceptance angle .theta. is about 30 to 50.degree. and it is difficult to manufacture an optical fiber that can gather and transmit a large quantity of light.
To sum up, the conventional optical fiber has condensed a small quantity of light, and therefore, the quantity of light radiated from the side face of the optical fiber has been insufficient.